1. Field of the Invention
The present invention relates to an automatic tool changer for a precision roll turning lathe, and more particularly to an automatic tool changer for use in a roll turning lathe for performing ultra-precision machining of a mold roll for molding of an optical film.
2. Description of the Background Art
A liquid crystal panel for use e.g. in a flat TV is incapable of self-emission of light and, therefore, a backlight unit is used as a light source to transmit light to the liquid crystal panel from behind. A plastic sheet having a fine pattern formed in the surface, such as a prism sheet or a diffusion sheet, is used as a requisite part in such a backlight unit. Such a plastic sheet can be molded by allowing a sheet, which has been extruded from an extruder, to pass between roll molds each having a machined pattern in the surface.
Recent advancement of ultra-precision machining by means of a roll turning lathe has realized ultra-precision machining of a roll mold for molding of a prism sheet or a diffusion sheet, for use in a liquid crystal panel.
Roll turning lathes for performing such ultra-precision machining have various problems to be solved, which have not been so serious for conventional roll turning lathes, in order to satisfactorily perform ultra-precision machining.
In ultra-precision machining of a roll, a pattern of numerous fine three-dimensional shapes of the order of a few tens of μm to a few hundred μm is to be machined, and it takes a long time to complete machining for one roll. For example, in the case of a long roll for a plastic sheet, it takes about one minute to machine one longitudinal groove in, the axial direction of the roll. Because of the fineness of each groove or three-dimensional shape to be machined in such a roll, a vast number of grooves or three-dimensional shapes must be machined in the entire roll. Assuming that 30 thousand longitudinal grooves must be machined in the entire roll, it should take 30000 minutes, i.e. 500 hours or 3 weeks, to machine all the grooves without interruption in the roll.
The applicant has proposed a precision roll turning lathe provided with a linear motor-driven air slider to move a tool post linearly at a high speed in the longitudinal direction of a roll (patent documents 1 and 2). The precision roll turning lathe can speed up feeding of a cutting tool and thereby considerably reduce the time it takes to machine a roll. The reduction in the machining time of such a precision roll turning lathe can be made on the premise of non-interruptive machining.
With the recent trend toward larger-sized, higher efficiency liquid crystal panels, there is a demand for molding of wider sheets, which requires machining of a larger-sized roll mold. Thus, it is not uncommon these days to machine a roll mold for a prism sheet, having a long roll length such as two meters.
A roll is generally coated with electroless-plated nickel and therefore has a hard surface. A need therefore exits for a long-life cutting tool. At present, machining of a roll having a large machining area cannot be completed by using only one cutting tool, and a change of cutting tool is needed in the course of machining. It is common practice in conventional precision roll turning lathes to manually change a cutting tool after stopping the machine.
On the other hand, a composite processing machine is known which comprises a lathe which also functions as a machining center and which is provided with an automatic tool changer (see patent documents 3 to 5).